no Ping to WiFi connected raspberry pi

[hugostrange007]

Hello,

I am searching the web for a solution to this problem for some days now and didnt find, so
I decide to open this topic.
..another topic: https://bbs.archlinux.org/viewtopic.php?id=168075 is similar to what I got here, but I have given static IPs to all my devices.
and its well, another problem I think so forgive me this one.

my router is a PRGAV4202N2 (IP is 192.168.0.138, has Internet Connection)
my WiFi AccessPoint is a 3G/4G Wireless N Router Model No. TL-MR3420 (IP is 192.168.0.131, no Internet - only Internet over default Gateway 192.168.0.138)
my raspberry pi has IP 192.168.0.23 (WiFi)
my smartphone has IP 192.168.0.10 (WiFi)

I have installed VLC on the pi (active http controls) and want to use the VLCremote app in order to ..yeah remotely switch songs on my PA.
everything working fine while I connect the pi over my Gateways´ builtin WiFi.
as I turn off the Gateways´WiFi, and use the WiFi AccessPoint (TL-MR3420), the pi is connecting and I can ssh to the pi, its also found by 'Advanced IP Scanner' from my mainPC (IP 192.168.0.1) and the pi can then access Internet aswell.
only thing is, I cannot ping it from my mainPC and my Mobile Phone (192.168.0.10) also wont find the pi anymore.
as I try ping from PC, I get something like this: ..host not reachable (4 times, and then..) sent: 4 recieved: 4 lost:0

I tried forwarding several ports, on the Router and on the WiFi AccessPoint, but no success so far.
like I say ssh work fine, also I accessed the webinterface of vlc over my mainPC, so that works. its not on the pi I think its more about portforwarding or maybe my AccessPoint is just crap. maybe I forgot something..
help is very much what I look forward to at this point.